10 Reasons Why Your House Is So Humid With The AC On

why is my house so humid with the ac on

It’s the middle of summer, your AC is on, but your house is still humid.

This is a pretty common problem I run into as a licensed HVAC contractor and A/C humidity is one of the most important factors that change our comfort and health.

The most common reason for a humid air conditioner is a dirty HVAC filter that causes the coil to get very cold or even freeze releasing significant moisture into the air ducts.

A/C filters should be changed often and never allowed to restricts airflow.

Below are the top 10 reasons for a humid AC system…

  1. Oversized AC Unit 
  2. Clogged Air Filters
  3. Dirty Evaporator Coil
  4. Frozen Evaporator Coil
  5. Your AC Is Reaching It’s Life Expectancy
  6. Poor Insulation
  7. Very High Outdoor Humid Air
  8. You’re Showering or Cooking
  9. Your Home Is Overcrowded 
  10. Faulty Dryer Vents

Reason #1: Oversized AC Unit 

Having an oversized air conditioner is a very common reason why some homes have feel damp while running the AC.

If your system is too large it will cool the house very quickly and shut down before it has a chance to reduce the humidity.

This is often called “short cycling”. An air conditioner that is short cycling typically has about half the life expectancy of a properly sized unit.

A properly sized air conditioner should run for about 15-20 minutes per hour, turning on and off 2-3 times per hour.

During extreme heat, it may run continuously. These longer run cycles are more efficient and give the system a chance to remove the humid air, keeping you more comfortable at higher temperatures.

Read Also >> What Is Normal A/C Temperature?

Reason #2: Clogged Air Filters

dirty ac filter

Restricted airflow from a clogged filter is a frequent cause for high humidity.

Proper airflow is essential for your air conditioner to function effectively. I invite you to read my detailed guide on furnace filter change frequency here.

Clogged filters greatly reduce the airflow moving through the evaporator coil preventing it from exchanging heat properly.

This allows the coil to get very cold and usually freeze up, restricting even more airflow.

It may continue running and give you some cooling, but it will struggle to maintain on hot days and won’t reduce your humid environment to a comfortable level.

Luckily this issue can be easily corrected by simply replacing your air filter on a regular basis.

Reason #3: Dirty Evaporator Coil 

Similar to a clogged filter, if the fins on your evaporator coil are dirty they can’t properly exchange heat.

Lack of airflow from excess debris on your coil will cause it to freeze up and prevent it from cooling effectively. You can read our guide on the best evaporator coil cleaners here.

This also prevents the removal of moisture in the air causing the humidity in your home to stay at uncomfortable levels.

Not only does a dirty coil cause your home to be uncomfortable, but it greatly reduces your indoor air quality.

Mold and mildew can grow and create health issues for you and your loved ones.

Reason #4: Frozen Evaporator Coil

frozen ac coil

Whether it be from a lack of airflow or low refrigerant pressures, a frozen evaporator coil cannot successfully lower the excess humidity in your home.

As stated above, restricted airflow can cause your evaporator coil to freeze up, but low refrigerant pressures from a leaking system can also cause it to ice up.

A lack of refrigerant causes your system pressures to drop which also lowers the refrigerant temperature.

At a certain point, the temperature drops below freezing and the moisture that condenses on the coil turns to ice.

This will be a recurring problem that will need to be diagnosed and corrected by a professional to prevent further issues with your air conditioner.

Reason #5: Your AC Is Reaching It’s Life Expectancy

Air conditioners wear out over time.

Your AC may continue to come on and run, but its capacity to cool will diminish with age. The compressor begins to wear out and can’t compress and move the refrigerant effectively.

Indoor and outdoor coils may deteriorate, preventing proper heat exchange.

Unfortunately like everything else in this world, air conditioners aren’t built to last forever.

At some point, usually after about 15 years, you’ll have to consider replacing it to continue cooling your home efficiently and effectively.

If you have an older system and notice it just isn’t performing like it used to, it’s probably best to have it looked at.

Having regular maintenance done on a yearly basis can extend the life of your equipment and help you prepare for a replacement system when the time is right.

RELATED: How To Add Refrigerant To AC?

Reason #6: Poor Insulation

insulation reduces ac load

Insulation is the first line of defense against the heat of a scorching summer day.

A poorly insulated home that isn’t sealed very well will struggle to stay comfortable. Read our review on the best DIY spray foam to help seal up your home.

Allowing the humidity from outside to constantly make it’s way in will make your AC work extra trying to maintain temperature.

If you’re always adding more moisture to the air, it’s almost impossible to get rid of it fast enough.

Because of this, the relative humidity in your home will stay high even if the temperature is where you want it to be.

Insulating and sealing your home will help keep you comfortable as well as lowering your utility bills.

Reason #7: Very High Outdoor Humid Air

Sometimes your AC is working just fine and mother nature has other plans for your comfort.

When the humidity outside reaches oppressive levels, there’s only so much your system can do.

It will continue to reduce the dampness and temperature, but you may have to wait until the weather changes before it’s not so muggy again.

Keep an eye on the weather forecast and check the dew point. The higher the dew point, the more moisture there is in the air outside.

A dew point of 65 and higher is generally very humid and will feel uncomfortable.

Reason #8: You’re Showering or Cooking

Adding moisture to your home will cause your system to work overtime to remove the high humid air.

When we use the stove or oven, cooking or baking, we can increase the amount of moisture in the air with the steam from boiling water or foods.

The same is true for bathing and showering. The steam from the hot water enters the air and causes your home’s humidity to increase.

Try to use your bath fan and range hood when showering and cooking if you have them. This will help prevent excess moisture from being added to your home.

Reason #9: Your Home Is Overcrowded 

People make heat and moisture. Too many people create an uncomfortable space.

When you have a lot of people over, you are likely to increase the temperature and moisture level in your house.

Every person creates heat and moisture from their body such as from perspiration and breathing.

In fact, when sizing equipment for commercial applications I have to include the number of people that will occupy the space to account for the additional heat load.

It’s usually an additional 500-600 btu per person.

So if you notice your home feels sticky when it’s crowded, try increasing circulation with ceiling fans and slightly lower the temperature on your thermostat to allow it to run a longer cycle.

This should help lower the humidity.

Read Also >> My AC Does Not Shut Off When I Set The Temp At Reached?

Reason #10: Faulty Dryer Vents

If your dryer vent is clogged with lint or disconnected, it will add moisture from your damp laundry to the space around it.

Dryer vents should be made out of smooth metal (the duct inside the wall) and it should have a minimum number of turns. Check out our guide on how to fix dryer vents here.

The dryer vent should be as short as possible from the interior to the exterior to properly remove all of the moisture.

Get your dryer vent professionally cleaned every 1-2 years.

The Effects of a Humid A/C

Our bodies natural way of cooling down is to sweat.

The sweat evaporates and removes heat allowing our body to stay cool.

When we’re subjected to high humid air during the summer our bodies struggle to regulate temperature effectively and we feel uncomfortable because our sweat cannot evaporate quickly.

The same is true with indoor temperature. If the humidity is high inside when we’re running the AC, it will feel uncomfortable.

And if being uncomfortable isn’t enough, you also risk mold and mildew growth from the excess moisture. This can negatively affect your health and damage your home.


Having a properly sized AC is the biggest help in reducing damp air effectively.

Oversized and undersized air conditioners just can’t cut it when it comes to removing moisture properly. It’s also extremely beneficial to take care of your equipment.

Regular maintenance and filter changes will help keep you comfortable and extend the life of your equipment.

Sealing up your house and adding insulation will prevent excess heat and humidity from entering your home causing your system to work harder than it needs to.

It’s also very helpful to install and use bath fans and range hoods. The idea is to reduce the load on your AC. 

Every little bit helps, so you can even install a dehumidifier to help your system when the humidity is extreme.

RELATED: Why Does My Window AC Has Water In It?

Frequently Asked Questions

Why is the humidity not dropping?

Your AC may not be bringing the humidity down because it has poor airflow from clogged air filters or a dirty evaporator coil.

With that, the coil may even be iced up and cannot effectively remove moisture from the air.

Is there a setting to make the humidity to go down?

A properly sized air conditioner will remove humidity naturally. All you have to do is set the temperature to a comfortable setting. The Department of Energy recommends 78 degrees.

Should I turn on the air conditioner if there is high humidity?

Yes, turning on the AC when the humidity is high is recommended. High humidity levels are uncomfortable and encourage mold and mildew growth.

Should I drop the thermostat temperature to reduce moisture?

Lowering the temperature in your home can lower the humidity as well because it allows the air conditioner to run a longer cycle. Longer run cycles remove moisture from the air more effectively.

Is 70% humidity considered high?

Yes, 70% relative humidity in a house is high. Recommended humidity range is 30-60%.

While the recommended temperature to run your AC in the summer is 78 degrees, most people set their temperature around 74.

Keep in mind the best way to remove humidity is to allow your system to run longer cycles, so your temperature may be different depending on your home and what is comfortable to you.

How does the A/C work in lowering relative humidity?

In a nutshell, your air conditioner absorbs heat from the home and rejects it outside. The evaporator coil (indoor coil) is very cold and the blower from your furnace moves warm air through it to exchange heat. In the process, moisture in the air condenses on the coil and collects in a pan underneath to be drained away with a condensate line.

Think of how an ice cold glass of lemonade sweats on a hot summer day. This is the same idea, just on a larger scale.


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